Such an attachment has been disclosed in U.S. Pat. No. 3,082,716 in the name of Karl Nicolay, issued 26 Mar. 1963 to the assignee of my present invention, which also describes a mechanism for manually coupling the looper bar to the machine drive whenever such a padded workpiece comes to lie under the presser foot. As further shown in that patent, the looper bar is fastened to a block which is vertically slidable and axially swingable on the stem of the presser foot, the vertical position of that block and thus of the looper bar being controlled by a workpiece feeler via a yieldable linkage which allows the pressure foot to be lifted off the workpiece without rupturing the loose stitches enveloping the looper bar. The prior patent also shows a reciprocating bottom feeder overlain by a top feeder comprising a serrated gripper foot which is straddled by prongs of the presser foot above a work-supporting plate.
It has already been proposed to utilize a sensor for detecting the presence of an elastic shoulder pad to be stitched, characterized by a predetermined minimum thickness, as a means for automatically actuating a looper bar of this character. A problem heretofore encountered with such an arrangement lies in the difficulty of obtaining an accurate measure of the thickness of the compacted workpiece underneath the presser foot since the rise and fall of the bottom feeder, which periodically projects above the workpiece-supporting surface of a slotted stitch plate, changes the position of the presser foot which therefore may improperly signal the presence of padding when there is in fact only a minor change in fabric thickness.